Number 13 is a wonderful book, Signs & Emblems by Erhardt Steibner and Dieter Urban, published in 1984.
I have only ever seen it on blogs and Flickr
groups, but it looks like a great source of inspiration. There's just something nice about a book full of mono graphics and symbols.
I have found a few copies of this one for sale here
, but none are what I would call cheap, but I bet it's worth every penny.